Plasma display panels (referred to below as “PDPs”) have attracted attention as one type of display device used for a computer and a TV. Such a PDP is a flat display apparatus that makes use of radiation caused by gas discharge. With the PDP, high-speed and high-definition display is easily possible. Moreover, size enlargement or size and weight reduction of the PDP is easily realized. Accordingly, the PDP is widely used in fields such as video display apparatuses and public information display apparatuses (see Patent Literature 1). The PDP includes direct current (DC) and alternating current (AC) types. The AC PDP, in particular a surface discharge PDP, has a high technological potential in view of its lifetime properties and a large screen size, and therefore has been commercialized.
A conventional common AC PDP is mainly composed of a pair of substrates (i.e. front and back substrates) that are disposed in opposition to sandwich a discharge space therebetween. The front and back substrates are sealed together at the sealing portion formed along the peripheral edges thereof to enclose an inner space between the substrates. The sealing portion includes a sealing material, such as low-melting glass.
On a main surface of the front glass substrate which forms a base surface, an Ag paste is applied and baked to form display electrodes each having a pair of electrodes in a stripe pattern. Following that, a glass paste is applied over the front substrate glass on which the display electrodes have been formed, and baked to form a dielectric layer composed of lead oxide. On the surface of the dielectric layer, a MgO layer or a MgO-containing protective layer is formed by a sputtering method or the like.
On a main surface of the back glass substrate, the Ag paste is applied and baked to form address (i.e. data) electrodes in a stripe pattern. Following that, the dielectric layer is sequentially formed on the main surface of the back glass substrate by the same method as the above. Subsequently, barrier ribs in a stripe pattern are formed on the dielectric layer in a manner such that the barrier ribs partition each address electrode. The barrier ribs are formed by applying a glass paste on the dielectric layer and then baked. After the barrier ribs are formed, phosphor ink containing one of red (R), green (G) and blue (B) phosphors is applied to the lateral surface of each barrier rib and on the exposed surface of the dielectric layer between adjacent barrier ribs. The phosphor ink is then baked at 500° C. to remove a resin component from the paste, thus forming the phosphor layers.
Following that, a sealing material paste is applied along the peripheral edges of the main surface of the back substrate on which the phosphor layers have been formed. The sealing material paste is composed of a mixture of the sealing material, resin (binder), and solvent. The sealing material contains lead oxide-based glass and an oxide filler in a mixture. In the manufacturing process, the sealing material paste is heated by pre-baking, whereby organic components contained in the paste are removed to some extent. The front and back substrates are superposed and positioned, with the main surface of the front substrate on which the display electrodes have been formed opposing the main surface of the back substrate on which the address electrodes have been formed, so that the display electrodes intersect the address electrodes. With the front and back substrates positioned as mentioned above, baking (i.e. sealing) step is performed to form the sealing portion. This seals the inner space enclosed by both the substrates.
In the pair of substrates, top surfaces of the barrier ribs formed on the back substrate abut against the main surface of the front substrate. As a result, adjacent barrier ribs partition the inner space into discharge cells. Between adjacent barrier ribs is the discharge space. After the above-mentioned sealing step, the inner space enclosed by both the substrates is evacuated. Subsequently, a rare gas, such as a Ne—Xe-based or a Xe—He based gas, is enclosed as a discharge gas at a predetermined pressure (normally at 40 to 80 kPa).
In order to display an image on the PDP, the method employed is one that expresses gradations in an image by dividing one field of the image into a plurality of subfields (S.F.) (e.g. intra-field time division grayscale display method). At the time of driving of the PDP, the display and the address electrodes are supplied with a current at a predetermined timing, leading to discharge generated in the discharge space. Upon the discharge, the discharge gas is ionized, whereby vacuum UV lines (i.e. mainly, resonance radiation with a wavelength of 147 nm and molecular radiation with a wavelength of 173 nm) are generated in the discharge space. The phosphor layers are excited by the vacuum UV lines, whereby a visible light is emitted. Thus, color display is realized on the panel as a whole.
Due to recent diversification in PDP usage, various PDP standards exist. One specific example of these standards is a conventional standard (SD) panel having 852 horizontal scanning lines (in width direction) and 480 vertical scanning lines (in length direction). Another example is a high-definition (HD) panel having 1024 horizontal scanning lines and 768 vertical scanning lines. Besides, currently, a full high-definition (HD) panel which is capable of displaying an image of higher definition than the high-definition panel is manufactured, and other panels with even higher definition are under development.
Such a high-definition PDP requires increased number of pixels. For example, a 42 inch visual size full HD panel has 1920 pixels horizontally×1080 pixels vertically with a vertical cell pitch of approximately 0.16 mm. In an ultra-high-definition panel having a visual size of 50 inches, which offers higher definition than the full HD panel, the number of cells is as many as approximately 4000 discharge cells horizontally×2000 discharge cells vertically. In this case, the vertical cell pitch is as extremely small as 0.1 mm.
In order to achieve an excellent image display capability with use of the great number of small discharge cells, it is necessary to assure that necessary light emission by discharge is performed at predetermined timing. As one method for achieving this, it is known that emission luminance can be improved by increasing a partial pressure of Xe in the discharge gas (e.g. increasing the partial pressure of Xe in the Ne—Xe-based gas from conventional 10% or so to approximately 30%).
On the other hand, due to a recent demand for electrical appliances with a low electric power consumption, a PDP using only a low drive voltage is needed. However, the problem is that increasing the Xe partial pressure in the discharge gas as mentioned above often causes an increase in the discharge voltage, resulting in an increase in the electric power consumption. The increase in the drive voltage also brings about a need for a pressure-proof driver, which might lead to an increase in the cost for a drive circuit and so on. Furthermore, an increase in the discharge strength makes the protective layer exposed to the discharge more vulnerable to abrasion (i.e. decrease in sputtering resistance). This might result in a shorter product life of the PDP itself.
Some of conventional countermeasures for the above problems attempt to decrease the discharge voltage and reduce the electric power consumption by optimally maintaining secondary electron emission properties of the protective layer. The protective layer has properties of absorbing an impurity gas and therefore being easily deteriorated. The impurity gas includes organic impurities contained in the sealing material paste and others, such as various types of resin, solvating media, and the solvent, and the impurity gas, such as carbon dioxide and water vapor generated when the impurities are baked out in the pre-baking and the sealing steps (all of which are referred to below simply as “impurities”). Accordingly, an attempt has been made to maintain the secondary electron emission properties of the protective layer, by preventing the absorption of the impurity gas or by using a material which does not easily absorb the impurity gas in the protective layer.
Specifically, as disclosed in Patent Literatures 3 and 4, and Non-Patent Literature 3, a method for maintaining the secondary electron emission properties of the protective layer is proposed. In the method, the protective layer is made of a composite oxide film using alkaline rare earth metal oxides, such as SrO, CaO, and BaO, instead of MgO. Also, the discharge space is evacuated to high vacuum of approximately 1×10−4 Pa before the discharge gas is introduced, in order to remove the impurities from the discharge space. According to the conventional method, steps from the protective layer formation step to the sealing step are performed throughout in one of a dried air atmosphere, a dried N2 atmosphere, and a dried O2 atmosphere. This effectively prevents the impurities, such as H2O, from contaminating the protective layer.
Patent Literature 5 also discloses a method of forming the protective layer made of the composite oxide film using the alkaline rare earth metal oxides, such as SrO, CaO, and BaO, and performing the sealing and the evacuating steps throughout in vacuum for the purpose of preventing unwanted absorption or reaction between the protective layer and H2O, CO, and CO2 contained in the atmosphere and effectively evacuating the impurities contained in the discharge space.